The hybrid tea rose, Rosa hybrida, is one of the most popular of cultivated plants. As with any valuable plant species, breeders are always working to improve the existing varieties; among the characteristics in which breeders of cut flowers are interested are color, morphology, fragrancy and length of vase life of the cut flower. Despite active efforts however, improvement in roses is slow and difficult to achieve through traditional breeding methods because of its perennial nature and a high degree of sterility caused by abnormal chromosome numbers. Tissue culture often provides a natural source of variation, as well as a convenient medium in which mutagenesis can be carried out. Moreover, in vitro transformation can be used as a tool for plant improvement, provided regeneration of the transformed plants can be achieved.
Notwithstanding the desirability of having reliable tissue culture methods for rose regeneration, there has been no success in regeneration of hybrid tea roses through the somatic embryogenesis process from tissue explants or single cells in liquid culture. The earliest rose tissue cultures were based on seed embryo cultures, in which the outer coat of a seed is removed, and the water-impermeable inner seed coat penetrated with a needle to permit seed germination in vitro of the sexual embryo germination medium (Asen and Larsen, Penn. State Col. Prog. Rep., No. 4, 1951). The utility of this technique is that it allows the rescue of hybrid roses which might otherwise abort, and also increases the speed of germination in rose breeding programs.
More recently callus cultures have also been established by a number of laboratories. Both Jacobs et al. (S. Afr. J. Agric. Sci. 11: 673-678, 1968; Agroplantae 1: 179-182, 1969; Agroplantae 2: 25-28, 1970; Agroplantae 2: 45-50, 1970) and Wulster and Sacalis (Hort. Sci 15: 736--736, 1980) have studied the effects of growth regulators on callus. Khosh-Khui and Sink (Sci. Hortic 17: 361-370, 1982) have also determined a number of parameters which aid in the establishment of rose callus cultures. However, none of the cultures reported by any of these authors went further than the callus stage. Shoot primordia have been reported in long term rose callus cultures (Hill, Nature 216: 596-597, 1967), but no plants were ever developed from these primordia. The most recent success with rose tissue culture has been the report on organogenesis from culture of immature seed embryos (Burger et al., Plant Cell. Tissue and Organ Culture 21:147, 1990).
Suspension cultures from rose tissue have also reportedly been successfully established by several different laboratories (Tulecke and Nickell, Science 130: 863-864, 1959; Gamborg, Exp. Cell. Res. 50: 151-158, 1968; Nash and Davies, J. Exp. Bot. 23: 75-91, 1972). However, no capacity for plant regeneration has been observed in any of these cultures. These rose cell suspension lines have been used extensively for biochemical studies of in vitro plant cells.
A number of workers have also reported adventitious shoot formation from callus cultures and tissue explants. For example, Lloyd et al. (Euphytica 37: 31-36, 1988) describe the formation of shoots from callus of R. persica x xanthina, in the presence of 6-benzyl adenine (6-BA) and naphthalene acetic acid (NAA); however, no success was observed with Rosa hybrida. Axillary shoot development has also been noted by many authors as a means of rose micropropagation (Hasegawa, Hort. Sci. 14: 610-612, 1979; Kirvin and Chu, Hort. Sci. 14: 608-610, 1979; Khosh-Khui and Sink, J. Hortic. Sci. 57: 315-319, 1982).
As can be seen from the foregoing review, although there has been some success in rose tissue culture there has not yet been a successful effort in regenerating an entire rose plant from Rosa hybrida through a somatic embryogenesis process. The recovery of plants from the meristematic tissues of axillary buds is not a regeneration process but rather the development of pre-existing meristems. There is thus still a need for a reliable method of regeneration from nonmeristematic tissue, which will provide the flexibility needed for successful practice of plant manipulation techniques, e.g. in vitro genetic manipulation. In vitro genetic manipulation will include protoplast fusion and gene uptake among others.